Metal casting can be explained as the process of pouring molten metal into a shaped cavity in order to transform it into a desired shape upon solidification. The primary step of this process is the creation of a mold having a shaped cavity to define the exterior shape of the casting. In addition, a core can be placed in the mold cavity to create an internal space of a defined shape in the casting. As used herein, “mold” includes both molds and cores used to shape the casting. Molds can be prepared using various heat resistant materials, including without limitation, sand, which provides the primary advantage of cost effectiveness. The molds can be classified as permanent or non-permanent. Non-permanent molds are destroyed to reveal the casted metal structure (e.g. sand molds). This step is referred to as the shakeout process. The casting process offers many advantages over other shaping processes. It is inexpensive and provides significant flexibility in form and material composition of the casts. Achieving net near shape and integral castings leads to the reduction in the time and costs associated with any additional machining and assembly requirements.
The properties of a casting can be established through the material and process parameters utilized during mold-making, treatment of the molten material prior to casting, chemical composition and characteristics of the casted material, the rate of solidification, and additional heat treatment after solidification.
The selection of the mold material and mold making process plays a vital role in the casting's characteristics. Sand, being a refractory material, can withstand very high metal pouring temperatures, which imparts the required dimensional stability at elevated temperatures. Sand also provides an economical advantage over other comparable refractory materials. Dry sand molding requires the introduction of a binder system to hold the loose sand particles during the mold shaping process. The finished mold's physical, chemical and thermo-chemical properties depend on the sand-binder system interactions and the process for creating the mold.
Mold formation can be divided into two categories: patterned and patternless. A patterned mold acquires its shape through the deposition of the mold material around a defined shaped object known as a pattern. The primary concern with patterned mold making is the cost and time involved in creating the pattern with the help of various machining and tooling activities. On the other hand, patternless molds can be created directly from a digital drawing or model through the implementation of additive or subtractive manufacturing techniques, and are valuable for creation of design prototypes and small-lot parts.
Additive manufacturing of molds, also known as layered manufacturing, has attained acceptance for rapid prototyping and rapid manufacturing, integrating computer aided manufacturing, and providing for rapid creation of complex mold shapes with high precision and tolerance levels. Rapid prototyping refers to the production of prototypes directly from a design. It is used for the product development phase of a production process because it is able to impart the characteristics in close proximity to the finished product, which enables the further investigation and analysis of the product before finalizing the end product features. The rapid casting process defines the integration of traditional metal casting techniques with additive manufacturing approaches to achieve either functional prototypes or end products.
Subtractive manufacturing techniques include traditional processes such as cutting, milling, grinding and drilling, which remove material from the mold to form a cavity, or in the case of a core, form the correct structure, or increase the precision of the mold.
Various rapid casting solutions have been developed during the last few decades to implement concurrent engineering approaches for the development of functional prototypes and customized production of metal castings for applications in various sectors of manufacturing industries. These rapid casting solutions include unique production approaches, such as selective laser sintering, fused deposition modeling, stereolithography, 3-D printing and rapid tooling.